Seagrass Herbivory Levels Sustain Site-Fidelity in a Remnant Dugong Population

Herds of dugong, a largely tropical marine megaherbivore, are known to undertake long-dis-
tance movements, sequentially overgrazing seagrass meadows in their path. Given their
drastic declines in many regions, it is unclear whether at lower densities, their grazing is
less intense, reducing their need to travel between meadows. We studied the effect of the
feeding behaviour of a small dugong population in the Andaman and Nicobar archipelago,
India to understand how small isolated populations graze seagrasses. In the seven years of
our observation, all recorded dugongs travelled either solitarily or in pairs, and their use of
seagrasses was limited to 8 meadows, some of which were persistently grazed. These
meadows were relatively large, contiguous and dominated by short-lived seagrasses spe-
cies. Dugongs consumed approximately 15% of meadow primary production, but there was
a large variation (3–40% of total meadow production) in consumption patterns between
meadows. The impact of herbivory was relatively high, with shoot densities c. 50% higher
inside herbivore exclosures than in areas exposed to repeated grazing. Our results indicate
that dugongs in the study area repeatedly graze the same meadows probably because the
proportion of primary production consumed reduces shoot density to levels that are still
above values that can trigger meadow abandonment. This ability of seagrasses to cope per-
haps explains the long-term site fidelity shown by individual dugongs in these meadows.
The fact that seagrass meadows in the archipelago are able to support dugong foraging
requirements allows us to clearly identify locations where this remnant population persists,
and where urgent management efforts can be directed.

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